1. Field of the Invention
The present invention relates to a method of painting an object by reciprocating painting machines perpendicularly to the direction in which the object is conveyed and, more particularly, to a method of painting an object by reciprocating a plurality of painting machines arranged perpendicular to the direction in which the object is conveyed.
2. Description of the Prior Art
Numerous painting methods are known in which painting machines are reciprocated relative to a painted object that is conveyed at a predetermined constant speed. These known methods are disclosed, for example, in Japanese Patent Laid-Open Nos. 165064/1982, 255170/1985, 234970/1986, and 315361/1989.
Where an object having a large portion to be painted such as an automotive body is painted by reciprocating painting method, if one painting machine is used to cover the whole area of the painted portion of the object, then the speed at which the object is conveyed must be slowed down. This greatly deteriorates the productivity. Accordingly, the conventional method for painting an object having such a large painted portion has consisted in arranging plural, or two to four, painting machines perpendicularly to the direction in which the object is conveyed and reciprocating these machines in the direction in which they are arranged, for painting the object.
One kind of mechanism for reciprocating a painting machine transforms rotary motion of a rotating disklike cam into a rectilinear motion via a link and transmits the motion to the painting machine. This mechanism is generally adopted as a reciprocator, because it is simple in structure. Therefore, the painting machine draws a sinusoidal trajectory on a virtual painted surface while the object is being conveyed. At each turn of the trajectory of the reciprocating painting, the moving speed of the painting machine slows down, so that the amount of paint applied per unit area increases. Therefore, it is inevitable that the thickness of the paint film is increased at each turn. Especially, where plural painting machines are reciprocated to paint the object as described above, the orbits P' and Q' drawn by a pair of adjacent reciprocating painting machines which constitute a unit painting system are made to slightly overlap with each other at each turn as shown in FIG. 6; otherwise lack of hiding would tend to occur. Conversely, if the orbits are made to overlap with each other sufficiently, then the thickness of the paint film around the boundary A between the two adjacent orbits is considerably larger than the thickness of the paint film around the intermediate points A and C of the orbits P' and Q' as shown in FIG. 7. This is a major cause of nonuniformity in the paint film thickness.
The above-cited Japanese Patent Laid-Open No. 234970/1986 discloses techniques for varying the amount of delivered paint according to the changes in the applied area and varying the width of the painting pattern according to the amount of the delivered paint. Increases in the thickness of the paint film can be suppressed to some extent by controlling both amount of delivered paint at each turn of the painting orbit and width of the painting pattern, utilizing these techniques. However, it is very difficult to appropriately set the painting conditions such as the amount of delivered paint and the shaping air pressure. Especially, where a rotary atomizing electrostatic painting machine which applies paint efficiently is used, nonuniformity in the paint film thickness or lack of hiding often takes place unless the set conditions are maintained strictly. In this way, this method is not reliable.